You can install the second drive (any SATA drive will do). There is no pressing need to swap position of the drive physically, or to transfer your OS to the new drive. Both drives can work simultaneously.

What you may want to do is to move some of your data storage to the new drive -- its faster to have your data on a separate drive than your System. Search these forums or Google for instructions how to relocate your iPhoto and iTunes libraries. General data for most other programs can simply be dragged over to the new drive.

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